CHARLOTTE-MECKLENBURG STORM WATER DESIGN MANUAL
CHAPTER 4
STORM DRAINAGE SYSTEMS
4.1 Overview ............................................................4-1
4.1.1 Introduction ................................................4-1
4.1.2 Inlet Definition ............................................4-1
4.1.3 Criteria .......................................................4-1
4.2 Pavement Drainage ............................................4-2
4.2.1 Introduction ................................................4-2
4.2.2 Storm Drain Location .................................4-2
4.2.3 Inlet Types and Spacing ............................4-2
4.2.4 Longitudinal Slope .....................................4-3
4.2.5 Cross Slope ...............................................4-3
4.2.6 Curb and Gutter .........................................4-3
4.2.7 Median Ditches ..........................................4-3
4.2.8 Roadside Ditches.......................................4-3
4.2.9 Bridge Decks .............................................4-3
4.2.10 Median Barriers .......................................4-4
CHARLOTTE-MECKLENBURG STORM WATER DESIGN MANUAL
4.3 Gutter Flow Calculations ....................................4-4
4.3.1 Formula .....................................................4-4
4.3.2 Procedure ..................................................4-4
4.4 Grate Inlet Design ..............................................4-7
4.4.1 Grate Inlets on Grade ................................4-7
4.4.1.1 Design Steps..................................4-7
4.4.1.2 Example 1 ......................................4-9
4.4.2 Grate Inlets in Sag ...................................4-10
4.4.2.1 Type 'E' Grate ..............................4-10
4.4.2.2 Grated Drop Inlet..........................4-12
4.4.2.2.1 Example 2 ....................4-12
4.4.2.3 Open Throat Catch Basin ............. 4-13
4.4.2.3.1 Example .......................4-13
4.5 Combination Inlets ............................................4-14
4.5.1 Combination Inlets on Grade ...................4-14
4.6 Hydraulic Gradient ............................................4-14
4.6.1 Friction Losses ........................................4-14
4.6.2 Velocity Head Losses ..............................4-14
4.6.3 Entrance Losses ......................................4-14
4.6.4 Junction Losses .......................................4-15
4.6.5 Summary .................................................4-17
4.7 Storm Drain ......................................................4-20
4.7.1 Introduction ..............................................4-20
4.7.2 Design Criteria .........................................4-20
4.7.3 Capacity ..................................................4-20
4.7.4 Hydraulic Grade Lines .............................4-22
CHARLOTTE-MECKLENBURG STORM WATER DESIGN MANUAL
4.7.4.1 Design Procedure ........................4-22
4.7.5 Minimum Grade .......................................4-24
4.7.6 Design Procedures ..................................4-24
4.7.7 Rational Method Examples ......................4-27
CHAPTER 4 STORM DRAINAGE SYSTEMS
4.1 OVERVIEW
4.1.1 Introduction
In this chapter, guidelines are given for calculating gutter and inlet hydraulics and storm drainage
design. Procedures for performing gutter flow calculations are based on a modification of
Manning’s equation. Inlet capacity calculations for grate and combination inlets are based on
information contained in HEC- 22 (USDOT, FHWA, September 2009). Storm drain design is
based on the use of the rational formula.
4.1.2 Inlet Definition
There are four storm water inlet categories:
•
•
•
•
curb opening inlets
grated inlets
combination inlets
multiple inlets
In addition, inlets may be classified as being on a continuous grade or in a sag. The term
“continuous grade” refers to an inlet located on the street with a continuous slope past the inlet
with water entering from one direction. The “sag” condition exists when the inlet is located at a
low point and water enters from both directions.
4.1.3 Criteria
The following criteria shall be used for drainage system design.
•
Maximum spread of 6 feet in a travel lane based on a rainfall intensity of 4 inches/hour.
•
For a street with a valley gutter, another foot for the gutter is allowed with a total
maximum spread of 7 feet based on a rainfall intensity of 4 inches per hour.
•
For a street with a standard 2 feet 6 inch curb and gutter, an additional 2 feet is allowed
with a total maximum spread of 8 feet from the face of the curb based on a rainfall
intensity of 4 inches per hour.
•
See Section 2.3.1 for additional criteria on design storm frequencies.
Other
• In sag areas where relief by curb overflow is not provided the system standard design
level (Q25 – Q50) is to be used for analysis to ensure traffic flow is not interrupted (NC
Division of Highways, Guidelines for Drainage Studies and Hydraulic Design, 1999).
Therefore, in a sag condition where relief by overflow for a typical roadway cross section
is not provided, inlet capacity and the storm drainage system must be designed for:
o one dry 8 foot travel lane in the 25-year event for local streets; and
o one dry 8 foot travel lane in each direction in the 50-year event for
thoroughfares.
All upstream bypass flow must be considered in the design of the inlets and the storm
drainage system at the sag. Available overflow at sag locations can be considered when
making the determination for available travel lanes in the relevant storm events.
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CHARLOTTE-MECKLENBURG STORM WATER DESIGN MANUAL
•
For local roads the pipe system hydraulic grade line shall not surcharge sag inlets in the
25-year storm. For thoroughfare roads the pipe system hydraulic grade line shall not
surcharge in the 50-year storm.
•
When checking spread requirements at sag points (0% slope) check spread upstream of
sags (at the 0.5% slope point) to verify spread is not exceeded. Additional flanking inlets
upstream may need to be added to keep spread criteria from being exceeded at these
points.
•
Ponding at yard inlets outside the roadway shall be limited to a maximum of one foot
above a grated inlet for the 10-year storm.
•
No concentrated runoff flowing over City sidewalks except at driveways.
•
Roadside ditches, when allowed, shall be a minimum of 18 inches deep and shall provide
the capacity designed for a 10-year storm with 6-inches of freeboard. For subdivision
streets ditch flow for the 25-year storm shall not encroach onto the pavement. For
thoroughfare streets ditch flow for the 50-year storm shall not encroach onto the
pavement.
•
For subdivision streets the driveway and culvert shall be designed such that the flow from
a 25-year storm shall not encroach onto the roadway pavement. For thoroughfare streets
the driveway and culvert shall be designed such that the flow from a 50-year storm shall
not encroach onto the roadway pavement.
4.2 PAVEMENT DRAINAGE
4.2.1 Introduction
Design factors to be considered during gutter, inlet and pavement drainage calculations include:






Return period
Spread
Storm drain location
Inlet types and spacing
Longitudinal slope
Shoulder gutter





Cross slope
Curb and gutter sections
Roadside and median ditches
Bridge decks
Median barriers
4.2.2 Storm Drain Location
For standards related to storm drain location refer to the Charlotte Land Development Standards
Manual.
4.2.3 Inlet Types and Spacing
Inlet types shall be selected from the Charlotte Land Development Standards Manual or
equivalent North Carolina State Department of Transportation standards. Inlets shall be located or
spaced in such a manner that the design curb flow does not exceed the spread limitations. Flow
across intersecting streets will be reviewed and approved on a case by case basis.
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CHAPTER 4 STORM DRAINAGE SYSTEMS
4.2.4 Longitudinal Slope
A minimum longitudinal gradient is more important for a curbed pavement, since it is susceptible
to stormwater spread. Flat gradients on uncurbed pavements can lead to a spread problem if
vegetation is allowed to build up along the pavement edge.
Curb and gutter grades that are equal to pavement slopes shall not fall below 0.5 percent.
Minimum grades can be maintained in very flat terrain by use of a sawtooth profile. For long
vertical curves, cross slope may be varied slightly to achieve 0.5 percent minimum gutter grade.
4.2.5 Cross Slope
Refer to the design standards for pavement cross slopes as shown in the Charlotte Land
Development Standards Manual.
4.2.6 Curb and Gutter
Curb and gutter installation shall be designed in accordance with the relevant standards for the
jurisdiction.
4.2.7 Median Ditches
Large median areas and inside shoulders should be sloped to a center swale, preventing drainage
from the median area from running across the pavement. This is particularly important for highspeed facilities, and for facilities with more than two lanes of traffic in each direction.
4.2.8 Roadside Ditches
Roadside ditches (when allowed) will be required behind the shoulder of roadways without curb
and gutter to convey storm drainage away from the pavement to a discharge point. The steepest
side slope allowed is 3:1 (horizontal to vertical) on the roadside of the ditch and 2:1 on the side
closest to the right-of-way line. The ditch shall be graded to a minimum longitudinal slope of 1
percent and a maximum velocity of 4 ft/sec. For grass lined channels with velocities up to 7
ft/sec, permanent matting may be approved on a case by case basis. For velocities greater than 7
ft/sec, a concrete lined ditch may be required. Riprap will not be allowed for stabilization within
the street right-of-way (except as outlet protection on culverts).
In addition to the design of roadside ditches, a design shall be provided for driveway culverts for
each individual lot on the plan. The use of a small driveway culvert, 15 inches minimum
diameter, in conjunction with overtopping of the driveway itself will be allowed as further
described in Section 4.1.3. Sizes for all driveway culverts shall be shown in tabular form on the
plans, and each culvert shall be designed for the highest ditch flow applicable for the lot.
4.2.9 Bridge Decks
Drainage of bridge decks is similar to other curbed roadway sections. Because of the difficulties
in providing and maintaining adequate deck drainage systems, gutter flow from roadways should
be intercepted before it reaches a bridge. In many cases, deck drainage must be carried several
spans to the bridge end for disposal. Zero gradients and sag vertical curves should be avoided on
bridges. The minimum desirable longitudinal slope for bridge deck drainage should be 1 percent.
When bridges are placed at a vertical curve and the longitudinal slope is less than 1 percent, the
gutter spread should be checked to ensure a safe, reasonable design.
Scuppers are the recommended method of deck drainage because they can reduce the problems of
transporting a relatively large concentration of runoff in an area of generally limited right-of-way.
However, the use of scuppers should be evaluated for site-specific concerns. Scuppers should not
4-3
CHARLOTTE-MECKLENBURG STORM WATER DESIGN MANUAL
be located over embankments, slope protection, navigation channels, driving lanes, or railroad
tracks. Runoff collected and transported to the end of the bridge should generally be collected by
inlets and down drains.
For situations where traffic under the bridge or environmental concerns prevent the use of
scuppers, grated bridge drains should be used.
4.2.10 Median Barriers
Weep holes are often used to prevent ponding of water against median barriers (especially on
superelevated curves). In order to minimize flow across traveled lanes, it is preferable to collect
the water into a subsurface system connected to the main storm drain system.
4.3 GUTTER FLOW CALCULATIONS
4.3.1 Formula
The following form of Manning’s equation should be used to evaluate gutter flow hydraulics:
Q = [0.56/n] ST5/3 SL1/2 T8/3
Where:
Q = Gutter flow rate (ft3/sec)
n = Manning’s roughness coefficient
ST = Pavement cross slope (ft/ft)
SL = Longitudinal slope (ft/ft)
T = Width of flow or spread (ft)
Note: Manning’s n value for concrete curb and gutter is 0.016.
4.3.2 Procedure
Using Table 4-1, identify the following:
1. Inlet # – Assigned number (or label) of drainage structure.
2. Drainage Area – Area contributing runoff to the inlet (acres).
3. Surface ‘Q’ Sub. – Flow (in cfs) to the inlet.
Q = CiA
Where:
C = runoff coefficient for the sub-drainage area
i = 4.0 inches/hour
A = Area determined in #2 (acres)
4. Surface ‘Q’ Total = Qsub + ∑Qbypass
4-4
(4.1)
CHAPTER 4 STORM DRAINAGE SYSTEMS
5. Long. Slope – Longitudinal gutter slope at “Inlet #” (in feet per foot). Generally, this is
equivalent to the roadway centerline profile.
6. Trans. Slope – Transverse slope at “Inlet #” (in feet per foot). This is equivalent to the
roadway cross-slope.
7. K – This coefficient is used to determine the inlet capacity of a catch basin grate on
grade. Refer to Figure 4-1.
8. Inlet Capacity (cfs)
Qcap = KD5/3
Where:
D = ST x T, depth of flow at curb (ft)
For a “normal crown” street, ST = 3/8” per 1’ = 0.0313 ft/ft.
The maximum spread, T, is 8 feet. Therefore D = 0.0313 x 8 = 0.25ft.
9. Spread = Width of flow (feet)
The maximum width of spread in a travel lane is 6 feet. Total allowable spread with
standard curb and gutter is 8 feet, and with valley gutter is 7 feet.
10. Bypass Q = Surface Qtotal – Inlet Qcap
11. Bypass to Inlet# = List Inlet# directly downstream (bypass destination).
Remember to add:
Bypass Q + Surface Qsub =Surface Qtotal
Note: Computer software for gutter flow analysis is acceptable. The computer printout
should contain the same information that is shown in Table 4-1.
4-5
4-6
SLOPE (FT/FT)
LONG. TRANS.
K
CHECKED BY
PROJECT LOCATION
RAINFALL INTENSITY = 4.0 IN/HR
INLET
DRAINAGE
SURFACE 'Q' (CFS)
#
AREA (ACRES) SUB.
TOTAL
COMPUTED BY
PROJECT NAME
INLET
QCAP (CFS)
SPREAD (FT)
INLET CAPACITY CHART
BYPASS
Q (CFS) TO INLET #
DATE
REMARKS
CHARLOTTE-MECKLENBURG STORM WATER DESIGN MANUAL
Table 4-1 Inlet Capacity Chart
CHAPTER 4 STORM DRAINAGE SYSTEMS
4.4 GRATE INLET DESIGN
4.4.1 Grate Inlets on Grade
Following is a discussion of the procedure for the design of grate inlets on grade. Figure 4-1 is
used for the design of grate inlets on grade using type ‘F’ and ‘G’ grates.
4.4.1.1 Design Steps
1. Determine the following input data: Q, SL, ST, and n
2. D is the depth of water (or head) in the gutter immediately upstream of the grate (in feet).
However, before this depth can be calculated, certain parameters must be set. In the case
of street design, it is undesirable to have the street inundated and impassable due to the
amount of runoff drainage down a given street. Therefore, the maximum allowable top
width of water flow, or spread, T, in the gutter and street must be regulated such that
flooding does not occur.
3. With the discharge rate, Q, known, T can be solved by applying the modified Manning’s
equation:
Q = (0.56/n)ZD8/3SL1/2
(4.2)
Since:
D = T(ST)
(4.3)
And:
ST = 1/Z
(4.4)
T can be derived:
T = [Qn(Z5/3)/0.56(SL1/2)]3/8
Where:
(4.5)
T = top width, or spread, of water flow (ft)
Q = discharge to the inlet structure (cfs)
Z = reciprocal of the transverse slope (ft/ft)
n = roughness coefficient
SL = longitudinal street slope (ft/ft)
ST = transverse street slope (ft/ft)
D=depth of water in gutter (ft)
4. To determine Qcap of inlet:
Qcap = KD5/3
Where:
D = depth of water in gutter, upstream from the grate (ft)
Qcap = discharge intercepted by grate (cfs)
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CHARLOTTE-MECKLENBURG STORM WATER DESIGN MANUAL
K= grate inlet coefficient
Note: For SL> 6%, use the curve for SL= 6%
Figure 4-1 Grate Inlet Coefficient – On Grade
4-8
CHAPTER 4 STORM DRAINAGE SYSTEMS
5. Once T is calculated and determined to be within its imposed limits, D can be calculated
as follows:
D = T/Z
Where:
(4.6)
D = depth of water in the gutter, upstream from the grate (ft)
6. The inlet capacity of the grate, Qcap, can then be determined by using Figure 4-1. If 100%
interception is not achieved, the overflow must be included in the next downstream inlet.
4.4.1.2 Example 1
A 10-year discharge of 4 cfs (3 cfs off-site and 1 cfs additional roadway drainage) drains to a
residential street, with standard curb and gutter, in sheet flow and is to be intercepted in a catch
basin midway down the street.
Given:
SL = 4% = 0.04 ft/ft
ST = 3.125% = 0.03125 ft/ft
Tmax allowable = 8 ft
Type G grate
Find:
Determine whether this runoff is excessive and whether it can be handled by a
single grate, type ‘G’, catch basin.
Solution:
1. Z = 1/ST = 1/0.03125 = 32
T = 4 (0.016) (32)5/3
0.56(0.04)1/2
3/8
T = 7.1 ft (which is less than the 8 ft available)
Since T is lower than the maximum allowable top width of 8 ft, the runoff in the street is
acceptable.
2. D = T/Z = 7.1/32 = 0.22 ft
3. From Figure 4-1, K = 28.5
Therefore: Qcap = KD5/3 = 28.5(0.22)5/3 = 2.3 cfs
(4.7)
4. Qbypass = 4.0 – 2.3 = 1.7 cfs
Thus, the grate will intercept 2.3 cfs and 1.7 cfs will continue downstream to another structure. If
the next downstream structure can handle this 1.7 cfs in addition to any additional runoff that
reaches this structure, then the design is adequate. If this additional 1.7 cfs will overload the next
downstream structure, additional storm drainage structure can be added upstream.
4-9
CHARLOTTE-MECKLENBURG STORM WATER DESIGN MANUAL
4.4.2 Grate Inlets in Sag
Because a grate inlet in a sag condition is subject to clogging, a curb opening is required as a
supplemental inlet. The capacity of a grate in a sag depends upon the area of the openings and the
depth of water at the grate. Figure 4-2 can be used to calculate the head or flow for standard drop
inlet grates, and Figure 4-3 for Type ‘E’ grates in sag or depressed conditions.
4.4.2.1 Type ‘E’ Grate
For a type ‘E’ grate, the weir equation will control to a depth (D) of 0.69 feet. Refer to Figure 4-3.
Because a depth of 0.69 feet could never be reached without flooding the street, only the weir
equation will be used for the analysis of a street sag condition.
1. Knowing the Surface Qtotal, solve for the required depth:
D = Surface Qtotal
3.3P
2/3
P = 6.94 ft. (single type ‘E’ grate)
P = 9.92 ft. (double type ‘E’ grate)
2. Check the spread:
T=
D
ST
D = depth solved in #1 (ft)
ST = transverse slope (ft/ft)
4-10
(4.8)
CHAPTER 4 STORM DRAINAGE SYSTEMS
Figure 4-2 Standard Drop Inlet Grate NCDOT Std. 840.16 (2012)
Figure 4-3 Type ‘E’ Grate NCDOT Std. 840.03 (2012)
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CHARLOTTE-MECKLENBURG STORM WATER DESIGN MANUAL
4.4.2.2 Grated Drop Inlet
For a grated drop inlet, both the weir and orifice equation should be analyzed:
(orifice)
D=
Surface Qtotal
(0.6) A (64.4)1/2
2
(4.9)
A = 3.66 sq. ft. (open area of standard drop inlet grate-NCDOT Std. 840.16 (2012))
(weir)
D=
Surface Qtotal
3.3P
2/3
(4.8)
P = 11.08 ft. (perimeter of standard drop inlet grate–NCDOT Std. 840.16 (2012))
Solve both equations 4.8 and 4.9 for D, or refer to Figure 4-3. The larger D controls.
4.4.2.2.1 Example 2
Using the data given for Example 1 in 4.4.1.2, calculate the flow intercepted by type ‘E’ grate in a
sag location.
Given:
Same data given for Example 1 in 4.4.1.2
Find:
Determine the depth of flow that can be intercepted by a type ‘E’ grade in a sag
location.
Solution:
1. From Example 4.4.1.2, Surface Qtotal = 4 cfs
2. Solve for D using Figure 4-3 or Equation 4-8
D=
4 cfs
3.3 (6.94 ft)
2/3
= 0.31 ft
3. Check the spread
T = D = 0.31 ft
St 0.03125 ft/ft
= 9.9 ft
4. This exceeds the maximum allowable spread of 8 ft. Install additional catch
basins above the sag to intercept the additional flow, or try a double catch basin
at the sag.
D=
4 cfs
3.3 (9.92 ft)
2/3
= 0.25 ft
Check the spread
T= D=
St
0.25 ft
0.03125 ft/ft
= 7.9 ft
This is less than the maximum allowable spread of 8 feet, so a double catch
basin will work.
4-12
CHAPTER 4 STORM DRAINAGE SYSTEMS
5. Finally, sag location would need to be evaluated in the appropriate storm (25year for local roads and 50-year storm for thoroughfare roads) to ensure a
minimum 8 foot passable lane for vehicles.
4.4.2.3 Open Throat (Slab Top) Catch Basin
Depth of flow for this type of inlet depends on how many sides will be open to flow and what the
total length the openings are (L). Typical height for openings for this type of structure is 6 in (0.5
ft)A. Both weir and orifice equations should be analyzed to find depth at the inlet:
(weir)
D = Surface Qtotal
(3.3) (L)
2/3
If D> inlet opening height (.5 ft) then use the orifice equation to find the driving head (H) needed
to pass the flow into the inlet instead
(orifice)
H=
[Surface Qtotal]2
(0.6) (.5) (L) (64.4)1/2
since the driving head (H) is drawn to the centerline of the height of the inlet simply adding 0.25
ft to H will yield depth of flow (D)
D = H + (0.5/2)
4.4.2.3.1 Example
A four sided slab top open throated drop inlet is planned for a backyard. Surface flow is 25 cfs to
this inlet. All four sides of the inlet have openings with each inlet side being 4 ft long (16 ft
total). Since the height for inlet openings is 6 in the preceding equations can be used.
Analyzing weir flow for this inlet:
D = (25/(3.3*16))2/3= 0.61 ft
Since this is greater than the inlet opening height of 0.5 ft the orifice equation must be used to
find depth of flow:
H = (25/(0.6*0.5*16*64.41/2))2= 0.42 ft
Adding .25 ft to H to find depth of flow to the invert of the opening
D = 0.42 ft + 0.25 ft = 0.67 ft
Depth of flow is 0.67 ft. Add required freeboard and grade accordingly to allow this ponding
depth.
A
- For inlets with inlet opening heights different than 0.5 ft adjust these equations accordingly.
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CHARLOTTE-MECKLENBURG STORM WATER DESIGN MANUAL
4.5 COMBINATION INLETS
4.5.1 Combination Inlets on Grade
On a continuous grade, the capacity of an unclogged combination inlet with the curb opening
located adjacent to the grate is approximately equal to the capacity of the grate inlet alone. Thus,
capacity is computed by neglecting the curb opening inlet and the design procedures should be
followed based on the use of Figure 4-1.
4.6 HYDRAULIC GRADIENT
4.6.1 Friction Losses
Energy losses from pipe friction may be determined by rewriting the Manning’s equation.
Sf = [Qn/1.486 A(R)2/3]2
Then the head losses due to friction may be determined by the formula:
Hf = SfL
Where:
Hf = friction head loss (ft)
Sf = friction slope (ft/ft)
L = length of outflow pipe (ft)
4.6.2 Velocity Head Losses
From the time storm water first enters the storm drainage system at the inlet until it discharges at
the outlet, it will encounter a variety of hydraulic structures such as inlets, manholes, junctions,
bends, contractions, enlargements and transitions, which will cause velocity head losses. Velocity
losses may be expressed in a general form derived from the Bernoulli and Darcy-Weisbach
Equations.
H = KV2/2g
Where:
(4.10)
H = velocity head loss (ft)
K = loss coefficient for the particular structure
V = velocity of flow (ft/s)
g = acceleration due to gravity (32.2 ft/s)
4.6.3 Entrance Losses
Following are the equations used for entrance losses for beginning flows.
4-14
Htm = V2/2g
(4.11)
He = KV2/2g
(4.12)
CHAPTER 4 STORM DRAINAGE SYSTEMS
Where:
Htm = terminal (beginning of run) loss (ft)
He = entrance loss for outlet structure (ft)
K = 0.5 (assuming square-edge)
(Other terms defined above)
4.6.4 Junction Losses
Incoming Opposing Flows
The head loss at a junction, H j1 for two almost equal and opposing flows meeting “head on” with
the outlet direction perpendicular to both incoming directions, head loss is considered as the total
velocity head of outgoing flow.
Hj1 = (V32) (outflow)/2g
Where:
(4.13)
Hj1 = junction losses (ft)
(Other terms are defined above.)
Changes in Direction of Flow
When main storm drain pipes or lateral lines meet in a junction, velocity is reduced within the
chamber and specific head increases to develop the velocity needed in the outlet pipe. The sharper
the bend (approaching 90o) the more severe this energy loss becomes. When the outlet conduit is
sized, determine the velocity and compute head loss in the chamber by the formula:
Hb = K(V2) (outlet)/2g
Where:
(4.14)
Hb = bend head loss (ft)
K = junction loss coefficient
Table 4-2 below lists the values of K for various junction angles.
Table 4-2
Values of K for Change in Direction of Flow in Lateral
K
0.19
0.35
0.47
0.56
0.64
0.70
Degrees of Turn (In Junction)
15
30
45
60
75
90 and greater
K values for other degree of turns can be obtained by interpolating between values.
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CHARLOTTE-MECKLENBURG STORM WATER DESIGN MANUAL
Several Entering Flows
The computation of losses in a junction with several entering flows utilizes the principle of
conservation of energy. For a junction with several entering flows, the energy content of the
inflows is equal to the energy content of outflows plus additional energy required by the collision
and turbulence of flows passing through the junction.
The total junction losses at the sketched intersections are as follows:
The following equation can be used to calculate these losses:
Hj2 = [(Q4V42) – (Q1V12) - (Q2V22) + (KQ1V12)]/(2gQ4)
Where:
(4.15)
Hj2 = junction losses (ft)
Q = Discharge (cfs)
V = horizontal velocity (ft/s)
(V3 is assumed to be zero)
g = acceleration due to gravity (32.2 ft/s2)
K = bend loss factor
Where subscript nomenclature is as follows:
Q1 = 90o lateral (cfs)
Q2 = straight through inflow (cfs)
Q3 = vertical dropped-in flow from an inlet (cfs)
Q4 = main outfall = total computed discharge (cfs)
V1, V2, V3, V4 = horizontal velocities of foregoing flows, respectively (ft/s)
Also assume: Hb = K(V12)/2g for change in direction.
No velocity head of an incoming line is greater than the velocity head of the outgoing line.
The water surface of inflow and outflow pipes in junction are level.
When losses are computed for any junction condition for the same or a lesser number of inflows,
the above equation will be used with zero quantities for those conditions not present. If more
directions or quantities are at the junction, additional terms will be inserted with consideration
given to the relative magnitudes of flow and the coefficient of velocity head for directions other
than straight through.
4-16
CHAPTER 4 STORM DRAINAGE SYSTEMS
4.6.5 Summary
The final step in designing a storm drain system is to check the hydraulic grade line (HGL) as
described in the next section of this chapter. Computing the HGL will determine the elevation,
under design conditions, to which water will rise in various inlets, manholes, junctions, and etc.
Figure 4-4 on the following page is a summary of energy losses which should be considered.
Following this in Figure 4-5 is a sketch showing the proper and improper use of energy losses in
developing a storm drain system.
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CHARLOTTE-MECKLENBURG STORM WATER DESIGN MANUAL
Figure 4-4 Summary of Energy Losses
Source: www.lincoln.ne.gov/city/pworks/watrshed/require/drainage/pdf/chapt32.pdf
4-18
CHAPTER 4 STORM DRAINAGE SYSTEMS
Figure 4-5 Energy and Hydraulic Grade Lines for
Storm Drainage Under Constant Discharge
Source: www.ct.gov/dot/lib/dot/documents/ddrainage/11.12.pdf
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CHARLOTTE-MECKLENBURG STORM WATER DESIGN MANUAL
4.7 STORM DRAIN
4.7.1 Introduction
After the tentative locations of inlets, drain pipes, and outfalls with tailwaters have been
determined and the inlets have been sized, the next logical step is the computation for the rate of
discharge to be carried by each drain pipe and the determination of the size and gradient of pipe
required to convey this discharge. This is done by proceeding in steps from upstream of a line to
downstream to the point at which the line connects with other lines or the outfall, whichever is
applicable. The discharge for a run is calculated, the drain pipe serving that discharge is sized,
and the process is repeated for the next run downstream. It should be recognized that the rate of
discharge to be carried by any particular section of drain pipe is not necessarily the sum of the
inlet design discharge rates of all inlets above that section of pipe, but as a general rule is
somewhat less than this total. It is useful to understand that the time of concentration is most
influential and as the time of concentration grows larger, the proper rainfall intensity to be used in
the design grows smaller.
For ordinary conditions, drain pipes should be sized on the assumption that they will flow full or
practically full under the design discharge but will not be placed under pressure head. The
Manning’s equation is recommended for capacity calculations.
4.7.2 Design Criteria
The standard maximum and minimum slopes for storm drains should conform to the following
criteria:
1. The maximum pipe velocity shall not exceed 20 feet per second, or 10 feet per second in
corrugated metal pipe.
2. The maximum discharge velocity at pipe outlets is 10 fps except for pipes greater than 48
inches in diameter.
3. The minimum allowable slope is 0.5 percent or the slope which will produce a velocity of
2.5 feet per second when the storm drainage system is flowing full, whichever is greater.
Systems should generally be designed for non-pressure conditions by using the Manning’s
equation. When hydraulic calculations do not consider minor energy losses such as expansion,
contraction, bend, junction, and manhole losses, the elevation of the hydraulic gradient for design
flood conditions should be at least 1.0 foot below surface inlet elevation. As a general rule, minor
losses should be considered when the pipe velocity exceeds 6 feet per second (lower if flooding
could cause critical problems). If all minor energy losses are accounted for, it is usually
acceptable for the hydraulic gradient to reach 6 inches below the grate elevation.
4.7.3 Capacity
Formulas for Gravity and Pressure Flow
The most widely used formula for determining the hydraulic capacity of storm drain pipes for
gravity and pressure flows is the Manning’s equation and it is expressed below:
V = [1.486 R2/3S1/2]/n
4-20
(4.16)
CHAPTER 4 STORM DRAINAGE SYSTEMS
Where:
V = mean velocity of flow (ft/s)
R = The hydraulic radius (ft) – defined as the area of flow divided by the wetted
flow surface or wetted perimeter
S = the slope of hydraulic grade line (ft/ft)
n = Manning’s roughness coefficient (see Table 4-3)
In terms of discharge, the above formula becomes:
Q = [1.486 AR2/3S1/2]/n
Where:
(4.17)
Q = rate of flow (cfs)
A = cross sectional area of flow (ft2)
For pipes flowing full, the above equations become:
Where:
V = [0.590 D2/3S1/2]/n
(4.18)
Q = [0.463 D8/3S1/2]/n
(4.19)
D = diameter of pipe (ft)
The Manning’s equation can be written to determine friction losses for storm drain pipes as:
Where:
Hf = (2.88n2V2L)/D4/3
(4.20)
Hf = (29 n2LV2)/(R4/3)(2g)
(4.21)
Hf = total head loss due to friction (ft)
L = length of pipe (ft)
g = acceleration due to gravity = 32.2 ft/sec2
4-21
CHARLOTTE-MECKLENBURG STORM WATER DESIGN MANUAL
Table 4-3
Manning’s n Values
Type of Conduit
Concrete Pipe & Boxes
Corrugated Metal Pipe &
Boxes Annular Corrugations
Corrugated Metal Pipes,
Helical Corrugations, Full
Circular Flow
Wall and Joint
Description
Manning’s n value
Good joints, smooth walls
68 by 13 mm
2- 2/3 by ½ inch corrugations
68 by 13 mm
2-2/3 by ½ inch corrugated, 24 inch plate width
0.012-0.013
0.024
0.021
Note: For futher information, consult Hydraulic Design of Highways Culverts, Federal Highway Administration, HDS
No. 5, page 33.
4.7.4 Hydraulic Grade Lines
In calculating the hydraulic grade line within a closed storm drainage system, all head losses shall
be computed to determine the water surface elevation within various structures.
The calculations are begun at the upstream or downstream opening, dependent upon whether the
pipe is in inlet or outlet control. If it is inlet control the hydraulic grade line is the headwater
elevation minus the entrance loss and the difference in velocity head. If the outlet controls, the tail
water surface elevation or 0.8 times the diameter of the pipe, wherever is higher, is the outlet
hydraulic grade line. Hydraulic grade lines will be required only as requested on a case by case
basis.
If computer models are utilized then results should be consistent with the procedure outlined
below. All input data should be supplied, including loss coefficients, and output should be in
similar format to Figure 4-6.
4.7.4.1 Design Procedure
The head losses are calculated beginning from the downstream control point to the first junction
upstream, and the procedure is repeated for the next junction. The computation for outlet control
may be tabulated using Figure 4-6 and the following procedure:
1. Enter in Column 1 the station for the junction immediately upstream of the outflow pipe.
Hydraulic grade line computations begin at the outfall and are worked upstream taking
each junction into consideration.
2. Enter in Column 2 the outlet water surface elevation or 0.8 diameter plus invert out
elevation of the outflow pipe whichever is greater.
3. Enter in Column 3 the diameter (Do) of the outflow pipe.
4. Enter in Column 4 the design discharge (Qo) for the outflow pipe.
4-22
CHAPTER 4 STORM DRAINAGE SYSTEMS
5. Enter in Column 5 the length (Lo) of the outflow pipe.
6. Enter in Column 6 the friction slope (Sf) in ft/ft of the outflow pipe. This can be
determined from the following formula:
Sf = {Qn/1.486AR2/3}2
(4.22)
7. Multiply the friction slope (Sf) in Column 6 by the length (Lo) in Column 5 and enter the
friction loss (Hf) in Column 7.
8. Enter in Column 8 the velocity of the flow (Vo) of the outflow pipe.
9. Enter in Column 9 the contraction loss (Ho) by using the formula:
Ho = 0.25 (Vo2)/2g
(4.23)
Where: g = 32.2 ft/s2
10. Enter in Column 10 the design discharge (Qj) for each pipe flowing into the junction.
Neglect lateral pipes with inflows of less than ten percent of the mainline outflow. Inflow
must be adjusted to the mainline outflow duration time before a comparison is made.
11. Enter in Column 11 the velocity of flow (Vi) for each pipe flowing into the junction (for
exception see Step 10).
12. Enter in Column 12 the product of (Qi x Vi) for each inflowing pipe. When several pipes
inflow into a junction, the line producing the greatest (Qi x Vi) product is the line which
will produce the greatest expansion loss (Hi). (For exception, see step 10).
13. Enter in Column 13 the controlling expansion loss (Hi) using the formula Hi = 0.35
(Vi2)/2g.
14. Enter in Column 14 the angle of skew of each inflowing pipe to the outflow pipe (for
exception, see Step 10).
15. Enter in Column 15 the greatest bend loss (H∆) calculated by using the formula:
H∆ = KVi2/2g
(4.24)
Where: K = the bend loss coefficient corresponding to the various angles of skew of the
inflowing pipes.
16. Enter in Column 16 the total head loss (Ht) by summing the values in Column 9 (Ho),
Column 13 (Hi) and Column 15 (H∆).
17. If the junction incorporates adjusted surface inflow of ten percent or more of the mainline
outflow, i.e., drop inlet, increase Ht by 30 percent and enter the adjusted Ht in Column 17.
18. If the junction incorporates full diameter inlet shaping, such as standard manholes, reduce
the value of Ht by 50 percent and enter the adjusted value in column 18.
4-23
CHARLOTTE-MECKLENBURG STORM WATER DESIGN MANUAL
19. Enter in Column 19 the FINAL H, the sum of Hf and Ht, where Ht is the final adjusted
value of the Ht.
20. Enter in Column 20 the sum of the elevation in Column 2 and the final H in Column 19.
This elevation is the potential water surface elevation for the junction under design
conditions.
21. Enter in Column 21 the rim elevation or the gutter flow line, whichever is lowest, of the
junction under consideration in Column 20. If the potential water surface elevation
exceeds the rim elevation or the gutter flow line, whichever is lowest, adjustments are
needed in the system to reduce the elevation of the H.G.L.
22. Repeat the procedure starting with Step 1 for the next junction upstream.
4.7.5 Minimum Grade
The minimum allowable slope is 0.5 percent or the slope which will produce a velocity of 2.5 feet
per second when the storm drainage system is flowing full, whichever is greater.
The minimum slopes are calculated by the modified Manning’s equation:
S = (nV)2/(2.208 R4/3)
(4.25)
4.7.6 Design Procedures
The design of storm drain systems is generally divided into the following operations:
1. The first step is the determination of inlet location and spacing as outlined earlier in this
chapter.
2. The second step is the preparation of a plan layout of the storm drainage system
establishing the following design area:
a.
b.
c.
d.
Location of storm drains
Direction of flow
Location of manholes
Location of existing facilities such as water, gas, or underground cables
3. The design of the storm drain system is then accomplished by determining drainage
areas, computing runoff using the rational method, and computing the hydraulic capacity
using Manning’s equation. As a final step the hydraulic performance of the system can
be checked by calculating the hydraulic grade line.
4. The storm drain design computation sheet (Figure 4-6) and hydraulic grade line
computation form (Figure 4-7) can be used to summarize the design computations.
4-24
4-25
Figure 4-6 Storm Drainage Computation Form
Structure
PROJECT LOCATION
PROJECT NAME
From
To
Length (feet)
Drainage Area
'A' (Acres)
Increment
Total
A' X 'C'
Flow Time
(min)
CHECKED BY
COMPUTED BY
Velocity type
STORM DRAINAGE COMPUTATION FORM
Runoff Coefficient 'C'
Increment
Total
To Upper End
In Section
Rainfall Intensity (in/hr)
Total Runoff (cfs)
Pipe Diameter (in)
Full Capacity (cfs)
Flowing Full
Design Flow
Invert
Elevation
DATE
Upper End
Lower End
Manhole Invert Drop
Slope of Sewer (ft/ft)
CHAPTER 4 STORM DRAINAGE SYSTEMS
4-26
CHECKED BY
PROJECT LOCATION
Figure 4-7 Hydraulic Grade Line Computation Form
Final H = Hf + Ht
Sf = (Qn/1.486AR2/3 )2
Hf = Sf X Lo
Ho = 0.25(Vo2 )/2g
Hi = 0.35 (Vi2 /2g)
Hf = K(Vi2 /2g)
Junction Losses
Outlet
Water
Surface
Sf
Hf V o Ho Qi
Vi
Elevation Do Qo Lo
Station
(ft)
(ft) (cfs) (ft) (ft/ft) (ft) (ft/s) (ft) (cfs) (ft/s) QiV i
2
5
6
7
12
1
3 4
8
9 10 11
DATE
Ht = Ho + Hi + Hf
60 K = 0.56
15 o K = 0.19
30 o K = 0.35
75 o K = 0.64
o
45 o K = 0.47
90 o K = 0.70
Inlet
Water
Surface
Rim
V i2 Hi Angle Hf Ht Ht1.3 Ht0.5 Final H Elevation Elevation
2g (ft) ( o) (ft) (ft) (ft) (ft) (ft)
(ft)
(ft)
(ft) 13 14 15 16 17 18
20
19
21
COMPUTED BY
PROJECT NAME
HYDRAULIC GRADE LINE COMPUTATION FORM
CHARLOTTE-MECKLENBURG STORM WATER DESIGN MANUAL
CHAPTER 4 STORM DRAINAGE SYSTEMS
4.7.7 Rational Method Examples
The following example will illustrate the hydrologic calculations needed for storm drain design
using the rational formula (see Hydrology Chapter for Rational Method description and
procedures). Figure 4-8 shows a hypothetical storm drain system that will be used in this
example. Following is a tabulation of the data needed to use the rational equation to calculate
inlet flow rate for the seven inlets shown in the system layout.
Table 4-4
Hydrologic Data
Drainage
Time of
Rainfall
Area
Concentration
Intensity
a
(inches/hour)
Inlet (acres) (minutes)
1
2
3
4
5
6
7
a
b
2.0
3.0
2.5
2.5
2.0
2.5
2.0
8
10
9
9
8
9
8
Runoff
Coefficient
Inlet
b
Flow Rate
(cfs)
.9
.9
.9
.9
.9
.9
.9
11.3
15.8
13.6
13.6
11.3
13.6
11.3
6.26
5.84
6.04
6.04
6.26
6.04
6.26
Inlet and storm drain system configuration are shown in Figure 4-8
Calculated using the Rational Equation (see Chapter 2, Hydrology)
The following table shows the data and results of the calculation needed to determine the design
flow rate in each segment of the hypothetical storm drain system.
Table 4-5
Storm Drain System Calculations
Storm Drain
Segment
I1-M1
I2-M1
M1-M2
I3-M2
I4-M2
M2-M3
I5-M3
I6-M3
M3-M4
I7-M4
M4-O
Tributary
Area
(acres)
Time of
Concentration
(minutes)
Rainfall
Intensity
(inches/hour)
Runoff
Coefficient
Design
Flow Rate
(cfs)
2.0
3.0
5.0
2.5
2.5
10.0
2.0
2.5
14.5
2.0
16.5
8
10
10.5
9
9
11.5
8
9
13.5
8
14.7
6.26
5.84
5.76
6.04
6.04
5.60
6.26
6.04
5.27
6.26
5.08
.9
.9
.9
.9
.9
.9
.9
.9
.9
.9
.9
11.3
15.8
25.9
13.6
13.6
50.4
11.3
13.6
68.8
11.3
75.4
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CHARLOTTE-MECKLENBURG STORM WATER DESIGN MANUAL
Figure 4-8 Hypothetical Storm Drain System Layout
Source: www.lincoln.ne.gov/city/pworks/watrshed/require/drainage/pdf/chapt32.pdf
4-28